1. Field of the Invention
The present invention relates to a conductive gasket as may be used in applications requiring electromagnetic interference (EMI) shielding. More particularly, the present invention relates to an EMI gasket that has a relatively thick metal conductive layer and a flexibility that resists cracking when bent.
2. Description of Related Art
Many modern electronic devices emit or are sensitive to electromagnetic interference (EMI) at high frequencies. Electromagnetic interference is understood to mean undesired conducted or radiated electrical disturbances from an electric or electronic apparatus, including transients, which can interfere with the operation of other electrical or electronic apparatus. Such disturbances can occur anywhere in the electromagnetic spectrum. Radio frequency interference (RFI) refers to disturbances in the radio frequency portion of the spectrum but often is used interchangeably with electromagnetic interference. Both electromagnetic and radio frequency interference are referred to hereafter as EMI.
A shield usually is inserted between a source of EMI and a desired area of protection. A shield is defined as a metallic or otherwise conductive configuration, which has the capability of absorbing and/or reflecting EMI and reducing the energy levels of the EMI. Such shields normally take the form of a conductive housing, which is electrically grounded. A shield may be provided to prevent EMI radiating from a source or to prevent EMI (generated randomly or by design) from reaching a target, or both.
Most such housings are provided with access panels, hatches, doors or other removable closure. Any gap between the metal surface confronting, abutting or mating with the closure affords an opportunity for the passage of EMI. Gaps also interfere with electrical currents running along the surfaces of the cabinets from EMI energy, which is absorbed and is being conducted to ground. The gaps reduce the efficiency of the ground conduction path and may even result in the shield becoming a secondary source of EMI leakage from gaps acting as slot antennae. Accordingly, it is common to use a conductive seal or gasket between such surfaces to block the passage of EMI.
Various configurations of gaskets have been developed to close the gaps between components of the shield. These gaskets establish as continuous a conductive path as possible across any gap that may exist, for example, between cabinet components. A common gasket employs a flexible core enclosed in a woven fabric made at least in part with conductive fibers. Examples of such fabrics are disclosed in U.S. Pat. No. 4,684,762.
Another common gasket construction as disclosed, for example, in U.S. Pat. Nos. 4,857,668, and 5,597,979 has a flexible core enclosed in an electrically conductive sheath formed of a non-conducting woven or non-woven fabric. The fabric is rendered conductive by an electroless plating process wherein the fabric is dipped in a silver nitrate bath to impregnate the fabric with silver. In an alternative process, the conductive material including silver or copper may be applied by sputter deposition. After impregnation or coating with silver, the fabric is coated with a non-corrosive material to prevent the oxidation of the silver surface. Suitable coating materials applied either by electroplating or sputter deposition include a pure metal such a nickel or tin, a metal alloy such as Inconel® or Nichrome® or a carbon compound.
In still another type of gasket, the conductive sheath as shown in U.S. Pat. No. 6,541,698 comprises a polymeric film that is rendered conductive by the vapor deposition of a conductive metal onto a surface of the film.
There are several criteria used to measure the performance of an EMI gasket. For example, its electrical performance is measured by the surface resistivity in ohms/square of the gasket at a given compressive load. A low resistivity is desired as this means that the surface conductivity of the gasket is high. EMI shielding performance as measured in decibels over a range of frequencies ranging form 20 MHz to 18 GHz wherein a constant decibel level over this range is preferred. These performance criteria are dependent, in part by the thickness of the conductive metal component of the gasket sheath; a thicker layer providing better performance than a thinner layer.
A fabric or film rendered conductive as described above represents a material of choice because its flexibility allows the material to be wrapped around a compressible core. A disadvantage of such materials is that they are costly to manufacture and there are practical limits to the thickness of a metal layer that can be deposited either by vapor deposition or by electroless plating. For example the practical limit for metal thickness using electroless deposition is about 0.03 mils and for vapor deposition about 0.02 mils.
Another drawback of fabrics coated with metal by electroless or vapor deposition is that they tend to be porous. It is believed that this porosity may allow high frequencies to pass through the porous fabric. While porosity is not an issue with a metal-coated film, the issue of limits as to the thickness of the metal coating that is practicable with electroless or vapor deposition remains.
An alternative to such a fabric is to use a metal foil. A metal foil is less costly than a metallized fabric and foils are generally thicker. However metal foils per se are not compressible and therefore a metal foil gasket is not generally used between mating parts. Nor are metal foils generally used for wrapping around a compressible core because a foil is relatively stiffer than a metallized fabric or film and may crack if sharply bent. Consequently, the foil is subject to cracking or wrinkling when a gasket made with foil is repeatedly compressed and decompressed. In addition, most foils will corrode and are not galvanically compatible with other metals so that the use of foils, as a gasket sheathing material, is limited.
U.S. Pat. No. 3,555,168 discloses an RF shielding gasket comprising a metal foil bonded directly to a compressively resilient foam backing. However the foil merely lies along one side of the foam backing and does not wrapped around the backing to completely enclose it .
While metallized fabrics/films and metal foils have received wide acceptance in the industry, there still is a need for improvement of the materials used to sheath a compressible core of the gasket. In this respect gasket sheathing materials that have a relatively thick metal layer while being relatively flexible and inexpensive to manufacture would satisfy both performance and cost considerations.